Butyl rubber is understood to be a copolymer of an isoolefin and one or more, preferably conjugated, multiolefins as comonomers. Commercial butyl comprises a major portion of isoolefin and a minor amount, not more than 2.5 mol %, of a conjugated multiolefin. Halogenated butyls are also well known in the art, and possess outstanding properties such as oil and ozone resistance and improved impermeability to air. Commercial halobutyl rubber is a halogenated copolymer of isobutylene and up to about 2.5 mol % of isoprene.
Butyl rubber or butyl polymer is generally prepared in a slurry process using methyl chloride as a vehicle and a Friedel-Crafts catalyst as part of the polymerization initiator. The use of methyl chloride is advantageous because AlCl3, a relatively inexpensive Friedel-Crafts catalyst, is soluble in methyl chloride, as are the isobutylene and isoprene comonomers. Additionally, the butyl rubber polymer is insoluble in the methyl chloride and precipitates out of solution as fine particles. The polymerization is generally carried out at temperatures of about −90° C. to −100° C. See U.S. Pat. No. 2,356,128 and Ullmanns Encyclopedia of Industrial Chemistry, volume A 23, 1993, pages 288-295. Low polymerization temperatures are required in order to achieve molecular weights which are sufficiently high for rubber applications.
Raising the reaction temperature or increasing the quantity of isoprene in the monomer feed results in poorer product properties, in particular, in lower molecular weights. However, a higher degree of unsaturation would be desirable for more efficient crosslinking with other, highly unsaturated diene rubbers (BR, NR or SBR).
The molecular weight depressing effect of diene comonomers may, in principle, be offset by lower reaction temperatures. However, in this case the secondary reactions, which result in gelation, occur to a greater extent and these processes are more costly. Gelation at reaction temperatures of around −120° C. and possible options for the reduction thereof have been described (c.f. W. A. Thaler, D. J. Buckley Sr., Meeting of the Rubber Division, ACS, Cleveland, Ohio, May 6-9, 1975, published in Rubber Chemistry & Technology 49, 960-966 (1976)). The auxiliary solvents such as CS2 required for this purpose are not only difficult to handle, but must also be used at relatively high concentrations. A further disadvantage associated with the use of CS2 lies in the fact that polymerization reactions of this type are homogeneous in nature. Consequently, there are significant increases in solution viscosity as the polymerization reaction proceeds. This in turn necessitates carrying out these polymerizations to lower conversions (i.e. lower amounts of polymer per unit volume of solvent and therefore a cost disadvantage) as high solution viscosities give rise to heat transfer problems.
It is furthermore known to perform gel-free copolymerization of isobutene with various comonomers to yield products of a sufficiently high molecular weight for rubber applications at temperatures of around −40° C. using pretreated vanadium tetrachloride (EP-A-1-818 476), a combination of nitro compounds and vanadium (EP-A-1 122 267) or zirconium compounds (WO-02/18460-A1) and others. The present invention operates in the absence of vanadium-, zirconium- and/or hafnium compounds.